Ras represents a group of closely related monomeric globular protein of 189 amino acids (21 kDa molecular mass) which is associated with the plasma membrane and which binds either GDP or GTP. Ras acts as a molecular switch. When Ras contains bound GDP it is in the resting or off position and is “inactive”. In response to exposure of the cell to certain growth promoting stimuli, Ras is induced to exchange its bound GDP for a GTP. With GTP bound, Ras is “switched on” and is able to interact with and activate other proteins (its “downstream targets”). The Ras protein itself has a very low intrinsic ability to hydrolyze GTP back to GDP, thus turning itself into the off state. Switching Ras off requires extrinsic proteins termed GTPase-activating proteins (GAPs) that interact with Ras and greatly accelerate the conversion of GTP to GDP. Any mutation in Ras which affects its ability to interact with GAP or to convert GTP back to GDP will result in a prolonged activation of the protein and consequently a prolonged signal to the cell telling it to continue to grow and divide. Because these signals result in cell growth and division, overactive Ras signaling may ultimately lead to cancer.
Structurally, Ras proteins contain a G domain which is responsible for the enzymatic activity of Ras—the guanine nucleotide binding and the hydrolysis (GTPase reaction). It also contains a C-terminal extension, known as the CAAX box, which may be post-translationally modified and is responsible for targeting the protein to the membrane. The G domain is approximately 21-25 kDa in size and it contains a phosphate binding loop (P-loop). The P-loop represents the pocket where the nucleotides are bound in the protein, and this is the rigid part of the domain with conserved amino acid residues which are essential for nucleotide binding and hydrolysis (Glycine 12, Threonine 26 and Lysine 16). The G domain also contains the so called Switch I (residues 30-40) and Switch II (residues 60-76) regions, both of which are the dynamic parts of the protein which are often represented as the “spring-loaded” mechanism because of their ability to switch between the resting and loaded state. The key interaction is the hydrogen bonds formed by Threonine-35 and glycine-60 with the γ-phosphate of GTP which maintain Switch 1 and Switch 2 regions respectively in their active conformation. After hydrolysis of GTP and release of phosphate, these two relax into the inactive GDP conformation.
The most notable members of the Ras subfamily are HRAS, KRAS and NRAS, mainly for being implicated in many types of cancer. However, there are many other members including DIRAS1; DIRAS2; DIRAS3; ERAS; GEM; MRAS; NKIRAS1; NKIRAS2; NRAS; RALA; RALB; RAP1A; RAP1B; RAP2A; RAP2B; RAP2C; RASD1; RASD2; RASL10A; RASL10B; RASL11A; RASL11B; RASL12; REM1; REM2; RERG; RERGL; RRAD; RRAS; RRAS2.
Mutations in any one of the three main isoforms of RAS (H-Ras, N-Ras, or K-Ras) genes are among the most common events in human tumorigenesis. About 30% of all human tumors are found to carry some mutation in Ras genes. Remarkably, K-Ras mutations are detected in 25-30% of tumors (FIG. 1). By comparison, the rates of oncogenic mutation occurring in the N-Ras and H-Ras family members are much lower (8% and 3% respectively). The most common K-Ras mutations are found at residue G12 and G13 in the P-loop and at residue Q61.
G12C is a frequent mutation of K-Ras gene (glycine-12 to cysteine). This mutation had been found in about 13% of cancer occurrences, about 43% of lung cancer occurrences, and in almost 100% of MYH-associates polyposis (familial colon cancer syndrome). However targeting this gene with small molecules is a challenge.